The discrepancy between numerous basic science, epidemiological and clinical studies that supported the potential benefit of hormone therapy in preventing age-associated cognitive impairment, and the recently published results from the Women's Health Initiative Memory Study (WHIMS), underscores the fact that our understanding of hormone neurobiology is incomplete. Since the mechanisms of gonadal steroid hormones in reproductive tissue may not be identical to that of the brain, a better understanding of how these hormones work in the brain is needed. To this end, we will systematically address the role of the newly discovered membrane progesterone receptor (mPR) in mediating the neuroprotective effects of progesterone. Further, we propose that the failure of the WHIMS to show positive effects of hormone therapy was due, in part, to the synthetic progestin used in the hormone therapy regimen, medroxyprogesterone acetate (MPA), and hypothesize that its lack of efficacy is due to important differences in its mechanism of action, relative to that of progesterone. Our hypotheses will be tested in primary cortical cultures (explants and dissociated neurons) in which we have confirmed the expression of the classical progesterone receptor (PR) and the mPR as well. We will use complementary pharmacological (cell membrane impermeable progestins), molecular (siRNA) and genetic tools (PRKO mice) to address the following aims: Aim 1: deterimine if the mPR mediates the effects of progesterone on the ERK/MAPK pathway (a pathway that we show is required for progesterone-induced protection);Aim 2: determine if PR-specific patterns of ERK phosphorylation may explain the difference between progesterone's and MPA's ability to protect;Aim 3: determine if the mPR mediates the effect of progesterone on cell survival;and Aim 4: determine if progesterone protects in an in vivo model of stroke, specifically, in animals whose expression of the classical PR has been disrupted (PRKO). The ovariectomized, "stroked" mouse will serve as a common model used by all projects within this Program of research and will enhance and facilitate the conceptual and experimental integration among projects. Collectively, the studies proposed in this project may identify the mPR as an important molecule to which future drug discovery efforts can target for the treatment or prevention of brain dysfunction resulting from age- or age-associated diseases like Alzheimer's Disease.